1. Field of the Invention
This invention relates to methods of making volumetrically efficient wound capacitors, and more particularly, to methods of progressively forming an elongated member of interconnected capacitors and shearing successive capacitors therefrom.
2. Background Art
It is well known that small capacitors are very important in the electronic industry and that circuits have required smaller and smaller size capacitors for differing applications. It has also been recognized that when electrostatic capacitors are needed and small size is a requirement that only ceramic capacitors have been available in the small sizes. For example multilayered ceramic capacitors are on the order of 0.090.times.0.190.times.0.190 in volume. However ceramic capacitors have had many disadvantages which have been recognized. For example, they have had problems with piezo-electric effects and silver migration. These losses resulting in capacitance losses in service are inversely proportional to the volumetric efficiency of the capacitor and proportional to the dielectric constant of the ceramic compound. Thus as the dielectric constant goes up the losses go up. Even with improved, more costly fabrication processes, these undesirable effects still have taken place.
It has been found that the foregoing undesirable characteristics are not found in film capacitors which have high insulation resistance, low dissipation factor, long term stability, self-healing capabilities and a failure mode which is an open not a short. However, film capacitors have heretofore not been commercially producible in the small sizes achievable with ceramic capacitors while still retaining the capacitive value and the voltage values of ceramic capacitors. The reason for this is that the metallized material that is conventionally being commercially wound is substantially wider than that which would be required for small size units. If a sufficiently narrow width film would be available, it has not yet been possible to wind it commercially to obtain such a narrow width. The prior art does show the cutting of film capacitors into variable portions but none of this was concerned with providing small volumetric size capacitors for maximum capacitance value.
Wound film capacitors are normally fabricated from strips or webs of a conductively coated dielectric formed into a tight coil. In winding the capacitor coil, two or more such webs are wound together with dielectric material between and around them so that the conductive material forms electrodes which are effectively insulated from each other. To start the formation of the coil, two conductive webs are mated so that one edge of the first electrode extends beyond the corresponding edge of the second, while the opposite edge of the second extends beyond the corresponding edge of the first. Thus, normally in winding the capacitor coil the two conductive webs are wound in a staggered arrangement with the intermediate dielectric material positioned between the electrodes. The wound capacitor coil thus has one extending edge of each electrode forming an end of the coil.
Wound capacitors fabricated from plastic or paper dielectric material and interleaved with metal foils acting as electrodes are in many respects satisfactory. However, such capacitors have certain disadvantages. The conductors or electrodes of one type of wound capacitor is a thin tin foil which is of low inherent mechanical strength, so that when the leads are soldered to the edges of the foil, care must be exercised using the capacitors to prevent tearing away of the leads. In an attempt to provide a more economical wound capacitor, aluminum foil has been used with resulting normal difficulty soldering lead wires to aluminum.
Another type of wound capacitor is that in which the conductive portions of the capacitor are formed from plastic tapes having a metalized coating on one side. Again a serious problem exists when wire leads are connected to the edges of the metallized plastic web in providing good electrical contact with the metallized coatings of the capacitor as well as in providing a sufficiently strong physical connection between the lead and the electrodes.
A review of the prior art shows that a number of attempts have been made to solve these problems. For example, Rayburn in U.S. Pat. No. 3,040,415 fabricates a capacitor from metal foil and plastic web; the foils being separated from each other by winding them between a wider dielectric plastic web so that the edges of the plastic web extend well beyond the edges of the metallic foils. Leads are attached to a capacitor wound in this manner by heating them to a sufficiently high temperature so that when one lead is placed against each end, the lead will melt through the extending edge of the plastic web, which in turn flows away and permits the heated lead to make contact with the edge of at least one of the metal foils. Upon cooling, the melted plastic fuses back into a solid portion locking the lead within the edge of the capacitor coil tightly against the edge of the metallic foil. When this is done it is found that there is usually sufficient metal foil material within the fused portion of the plastic material to provide an electrical contact between the lead and the conductive strips of the capacitor. However, this method of attaching the leads introduces problems of contamination into the lead conductor bond line resulting in a corresponding reduction in both the product uniformity and lifetime. Furthermore, in this construction the wire conductor does not make full electrical contact with all of the electrodes. Rather the output is a combination of both direct and inductive coupling with the result that the wound electrode may have a different potential depending on the nature of the coupling at a particular point. In addition, this type of construction is prohibitive for a metallized film construction since there would be insufficient metal within the fused portion of the plastic material necessary to yield electrical integrity.
In another approach, described by McGraw in U.S. Pat. No. 3,188,716, the webs are wound in the form of a hollow straw. The purpose of this design is for producing very small values of capacitance since only one single turn of wound material constitutes a final capacitor. Therefore, to produce an average film capacitor with this design having 100 wound turns or more of metallized film, it would require a machine having 100 individual rolls or more of metallized film-(with highly sophisticated metallized patterns not commercially available with present state of art)-making only one single turn around a mandrel. Another problem is that the capacitors are fairly large in size. Thus, they do not readily fit size requirements for capacitors which are to be employed as parts of printed circuit boards and other electronic devices where very small capacitors are needed.